1. Field of the Invention
The present invention relates to elastic wave resonators used in resonators, band-pass filters and the like and to ladder filters constructed using elastic wave resonators. More specifically the present invention relates to an elastic wave resonator in which an IDT electrode is apodization weighted, and also relates to a ladder filter and a duplexer.
2. Description of the Related Art
To date, elastic wave devices have been widely used as resonators and band-pass filters.
For example, an elastic wave resonator having the electrode structure illustrated in FIG. 9 is disclosed in PCT International Publication No. WO 2007/108269. In an elastic wave resonator 1001 illustrated in FIG. 9, a plurality of first electrode fingers 1002 and a plurality of second electrode fingers 1003 are arranged so as to be interposed between one another. One end of each of the plurality of first electrode fingers 1002 is connected to a first common electrode 1004 and the other end thereof extends toward a second common electrode 1005 side. One end of each of the plurality of second electrode fingers 1003 is connected to the second common electrode 1005 and the other end thereof extends toward the first common electrode 1004 side. Furthermore, an end of each of a plurality of first dummy electrode fingers 1006 is connected to the second common electrode 1005. The other ends of the first dummy electrode fingers 1006 face the leading ends of the first electrode fingers 1002 with gaps therebetween. Similarly, an end of each of second dummy electrode fingers 1007 is connected to the first common electrode 1004. The other ends of the second dummy electrode fingers 1007 face the leading ends of the second electrode fingers 1003 with gaps therebetween.
Here, apodization weighting is applied so that the crossing width of the IDT electrode changes in the elastic wave propagation direction. In the apodization weighting, there are two maximum values of crossing width in the surface acoustic wave propagation direction of the IDT electrode 1002. Therefore, the envelopes that enclose a crossing region, that is, a region in which the first and second electrode fingers 1002 and 1003 cross one another, form the shape of two connected diamonds.
In PCT International Publication No. WO 2007/108269, it is supposed that, as a result of this kind of weighting, not only is spurious due to transverse-mode ripples reduced but also the Q value at the anti-resonant frequency and the electric power handling capability can be increased.
In the elastic wave resonator described in PCT International Publication No. WO 2007/108269, since there are large non-crossing regions in which the first and second electrode fingers 1002 and 1003 do not cross one another in the IDT electrode, the excitation intensity of elastic waves is smaller than that in a normal IDT electrode. Furthermore, in portions in which there is such a large non-crossing region, the first and second dummy electrode fingers 1006 and 1007 are long and therefore the resistive loss of the dummy electrode fingers is large. Therefore, there is a problem with this elastic wave resonator in that the Q value at the resonant frequency is unsatisfactory. Furthermore, in a ladder filter constructed using this elastic wave resonator, although the electric power handling capability has been increased, it has not been possible to sufficiently lower the insertion loss.